Tailoring Noise Frequency Spectrum between Two Consecutive Second Derivative Filtering Procedures to Improve Liquid Chromatography−Mass Spectrometry Determinations

This paper reports a simple chemometric technique to alter the noise spectrum of a liquid chromatography−mass spectrometry (LC−MS) chromatogram between two consecutive second-derivative filter procedures to improve the peak signal-to-noise (S/N) ratio enhancement. This technique is to multiply one second-derivative filtered LC−MS chromatogram with another artificial chromatogram added with thermal noises prior to the other second-derivative filter. Because the second-derivative filter cannot eliminate frequency components within its own filter bandwidth, more efficient peak S/N ratio improvement cannot be accomplished using consecutive second-derivative filter procedures to process LC−MS chromatograms. In contrast, when the second-derivative filtered LC−MS chromatogram is conditioned with the multiplication alteration prior to the other second-derivative filter, much better ratio improvement is achieved. The noise frequency spectrum of the second-derivative filtered chromatogram, which originally contains frequency components within the filter bandwidth, is altered to span a broader range with multiplication operation. When the frequency range of this modified noise spectrum shifts toward the other regimes, the other second-derivative filter, working as a band-pass filter, is able to provide better filtering efficiency to obtain higher peak S/N ratios. Real LC−MS chromatograms, of which 5-fold peak S/N ratio improvement achieved with two consecutive second-derivative filters remains the same S/N ratio improvement using a one-step second-derivative filter, are improved to accomplish much better ratio enhancement, approximately 25-fold or higher when the noise frequency spectrum is modified between two matched filters. The linear standard curve using the filtered LC−MS signals is validated. The filtered LC−MS signals are also more reproducible. The more accurate determinations of very low-concentration samples (S/N ratio about 5−7) are obtained via standard addition procedures using the filtered signals rather than the determinations using the original signals.